Although the mainstream light sources for lighting currently are fluorescent lights and incandescent light bulbs, those using LEDs (light-emitting diodes) as the light source, compared to fluorescent lights or the like, have less consumption power, longer life span, excellent safety aspects such as not being hot to the touch, and moreover, excellent environmental aspects as they do not contain toxic substances such as mercury, such that they are anticipated to become the mainstream light sources for lighting in the near future.
White LEDs currently in use are constituted by combining a blue LED and a YAG:Ce (yellow); they bear the problem of having poor color rendering properties to show natural coloration, and in particular, when a red object or a human skin is illuminated with such currently used white LEDs, they cannot show the colors illuminated by natural light. Thus, as methods for improving the color rendering properties of such currently used white LEDs, constituting a white LED by combining a near-ultraviolet LED and three types of phosphor, red, green and blue, or combining a blue LED and two types of phosphor, red and green, has been examined, and SrGa2S4:Eu has been described as a green emitting phosphor used for such purposes (refer to Patent References 1, 2 and 3).    [Patent Reference 1] Japanese Patent Application Laid-open No. 2002-060747    [Patent Reference 2] Japanese Patent Application Laid-open No. 2007-056267    [Patent Reference 3] Japanese Patent Application Laid-open No. 2007-214579